The invention relates to a porous electrode structure for a gas sensor as well as a sensor arrangement.
Gas sensors are known from German patent DE 197 01 493 C1, on the basis of semi-conducting gallium oxide for detection of reducing gases, such as hydrocarbons, hydrogen or even of solvents. There the gallium oxide layer lies on two electrically separated electrodes. With the reaction of reducing gases on the gallium oxide layer, their electrical resistance changes, which represents a measure for the concentration of the gas to be measured (hereinafter xe2x80x9cmeasured gasxe2x80x9d). By a covering of the gallium oxide layer with a non-continuous layer of gold islands, an increase in sensitivity to carbon monoxide is attained. The production parameters of the sensitive layer are hard to reproduce and decisively influence the measuring result.
From German published patent application DE 195 35 381 A1, electrode materials and sensor arrangements are known for the detection of hydrocarbons on the basis of lanthanide or rare earth compounds, which can be operated with the aid of an amperometric as well as a potentiometric measuring principle. Disadvantageous is the high impedance of these electrodes owing to their low electrical conductivity and, for example, the low adhesive capacity of the material on a solid electrolyte. In the description, electrodes of gold or gold alloys are also mentioned as sensitive layers for hydrocarbons, which have serious disadvantages. In addition to low stability over time, these electrodes of gold or gold alloys have a memory effect, which is dependent upon the preceding gas stresses and temperature cycles and makes a constant calibration necessary.
Underlying the invention is the object of making available an electrode structure as well as a sensor arrangement for detecting hydrocarbons, which, aside from a low impedance, guarantees a high temporal stability and reproducibility of the measurement signal.
The objective is accomplished by a porous electrode structure having a conductive gold framework, which contains oxide components. Surprisingly, a determinable memory effect does not arise here, as it does with hydrocarbon sensors having electrodes of gold or gold alloys. For the electrode structure of the invention, it is important that this have an open, i.e., permeable, porosity, wherein the electrical conductivity of the gold framework must furthermore be given in all spatial directions. The oxide components contained in the conductive gold framework can preferably have gallium oxide (Ga2D3. The gallium oxide therein should assume a proportion of 10-50 wt. %, preferably 30 wt. %, relative to the conductive framework. The oxide components can also contain, however, one or more oxygen ion-conducting solid electrolyte materials.
In the proposed sensor arrangements with at least one oxygen ion-conducting solid electrolyte, one measuring electrode arranged on a measured gas side and a counter electrode arranged on a reference gas side, the measuring electrode has a porous electrode structure with a conductive gold framework, which contains oxide components. This arrangement for a gas sensor is suited for detecting hydrocarbons in a measured gas.
In this connection, various oxygen ion-conducting solid electrolytes with different doping materials and supplemental amounts are usable, as well as mixtures thereof.
It proves to be especially advantageous if the oxide components of the hydrocarbon-sensitive measuring electrode contain gallium oxide. Here, the gallium oxide should assume a proportion of 10-50 wt. %, preferably of about 30 wt. %, relative to the conductive gold framework.
The oxide components for the measuring electrodes can also contain one or more oxygen ion-conducting solid electrolytes. Thus, for example, the adhesion or the coefficient of expansion between the oxygen ion-conducting solid electrolyte used and the electrode sensitive to hydrocarbons can be improved or adapted, if these are partially made of the same oxygen ion-containing material as the solid electrolyte used.
The potential measured between the measuring electrode and the counter electrode represents, with the presence of oxygen and hydrocarbons in the measured gas, a mixing potential which arises on the basis of different oxygen and hydrocarbon partial pressures between the measuring gas space and reference gas space. A change in the oxygen partial pressure in the measured gas accordingly leads to a change in the potential between the measuring electrode and the counter electrode. In order to be able to establish these possible changes of oxygen partial pressure in the measured gas, and to be able to separate the changes resulting therefrom in the measurement signal from the signal components which indicate changes in the hydrocarbon partial pressure in the measured gas, a comparison with a signal of an oxygen sensor arranged in the vicinity is expedient. The potential measured here on the basis of the various oxygen partial pressures in the measured gas and the reference gas can be deducted from the mixed potential between the measuring electrode and the counter electrode. There result in this case the signal components for which the hydrocarbon components in the measured gas are responsible.
Thus, further electrodes or electrode pairs of other materials can be arranged on the oxygen ion-conducting solid electrolyte which, together with the oxygen ion-conducting solid electrolyte, make possible a determination of the concentration of additional gases contained in the measured gas. This is expedient for obtaining a comparison signal, for example for establishing changes in the oxygen partial pressure in the measured gas, and spares the use of additional sensor arrangements.
With respect to the operating temperature of the measuring electrode of the invention, and also with an additional use of the oxygen ion-conducting electrolyte of the arrangement, for example for an oxygen sensor, a solid electrolyte material with sufficient oxygen ion conductivity must be used. The ideal temperature range for the use of the above-described, hydrocarbon-sensitive measuring electrode in a sensor arrangement lies in the range of 600 to 700xc2x0 C. If this temperature range is not present or stably guaranteed at the place of operation, then the use of heater elements is necessary. It is advantageous here if an electric heater element is arranged directly on the oxygen ion-conducting electrolyte, whereby between solid electrolyte and heater element one or more insulating layers must be arranged. Here, attention must also be paid in particular to an electrical insulation between the heater element and the electrodes of the gas sensor. Of course, electric heater elements can also be arranged spaced from the oxygen ion-conducting solid electrolyte.